The invention relates to a film for a film capacitor which is formed from an electrically insulating carrier film with an electrically conductive layer applied thereon, the electrically conductive layer in the film capacitor forming an electrode and the carrier film forming a dielectric. Furthermore, the invention relates to a film capacitor embodied with such a film.
Capacitors are produced, for a wide capacitance range, inter alia as film capacitors, the dielectric comprising a plastic film. The electrodes are formed from two conductive metal surfaces. These two metal surfaces comprise either thin metal films or metal layers vapor-deposited onto the films. The latter capacitors have a self-healing effect, i.e., in the event of an electrical breakdown, the metal layer evaporates in the vicinity of the discharge or at a point provided for this in proximity thereto. The active region is reduced as a result of this, in which case the capacitor can still be used, although with a slightly reduced capacitance.
U.S. Pat. No. 5,136,462 and FR8911713 describe films for a film capacitor which, instead of a continuous metal coating, have a metal coating which is subdivided into individual segments. The individual segments have a low sheet resistance; they are isolated from one another by nonconductive, trench-like cutouts and are electrically connected to one another only via narrow current bridges. These narrow current bridges serve as protection devices which, in the event of an electrical breakdown, isolate the effected segment from the remaining segments, so that the damage caused by an electrical breakdown is limited to a single segment or just a few segments. A capacitor having such a segmented film corresponds to thousands of individual capacitors connected in parallel.
In the case of the segmented film capacitor of EP 0 813 213 A1, the cutouts between the segments are covered with a thin conductive layer. The latter serves to enlarge the active area and also to avoid excess field increases at the edges between electrode segments and uncoated cutouts. The task of the conductive cutouts is, in the case of an electrical breakdown in an electrode segment, to increase the impedance in the vicinity of said segment and to prevent the supply of energy. In this case, the larger the a real portion of the thin conductive layer is chosen to be relative to the electrode segments, the larger the equivalent series resistance of the film capacitor becomes, as a result of which the electrical losses and hence the temperature in the capacitor are increased. However, higher temperatures in the capacitor cause aging phenomena to be accelerated.
WO 94/19813 discloses a film capacitor having a dielectric film with continuous electrodes vapor-deposited thereon. The electrodes are deliberately made relatively thin and have a correspondingly high sheet resistance of 5 to 300xcexa9. An improved dielectric strength, or increased breakdown voltage, compared with the segmented film capacitor described previously, is obtained by virtue of the increased sheet resistance.
The invention is based on the object of providing a film for a film capacitor and a film capacitor provided with the film which are only slightly subject to aging phenomena.
The invention is distinguished by the fact that, in a capacitor film having an electrically conductive layer as electrode and a current path structure applied on said layer, the sheet resistance of the layer is chosen to be as large as possible and that of the current path structure is chosen to be as small as possible. The first-mentioned measure results in a reduced loss of capacitance in the case of a breakdown, and the second measure leads to a reduction of the total sheet resistance of electrode and current path structure. As a result, the ohmic losses of the capacitor film are reduced, and thus so is the heating of said film. Both of the measures mentioned thus have a positive effect on the aging phenomena.
The electrically conductive layer of the film according to the invention for a film capacitor has a relatively high sheet resistance. In the event of an electrical breakdown, this high sheet resistance limits the area region at which the electrically conductive layer evaporates, as a result of which the effective electrode area and hence the capacitance of the capacitor are only slightly impaired. In addition, the breakdown voltage is increased and, in the event of breakdown, locally less electrical energy is converted into heat. The provision of a current path structure according to the invention preferably in the form of main and auxiliary current paths results in a reduced total sheet resistance relative to the sheet resistance of the electrically conductive layer. This reduced total sheet resistance reduces the ohmic losses when current flows in the capacitor and hence the temperature increase in the capacitor and thereby has a positive effect on the thermally dictated aging of said capacitor.
The current path structure is preferably composed of the same electrically conductive material as the electrically conductive layer or of a different electrically conductive material than the electrically conductive layer and has a contour raised above the electrically conductive layer. The individual current paths, preferably designed in strip form, are specifically subdivided into main and auxiliary current paths and form electrode structures configured in strip form. The sheet resistance of the current path structure is advantageously reduced by at least a factor of 20 relative to that of the electrically conductive layer. This allows an a really small coverage of less than 10% of the electrode layer by the current path structure without losses in the total sheet resistance reduction sought. The narrower and/or less dense the current paths are made, the smaller, however, is the reduction of the self-healing (high-resistance) electrode area of the capacitor film. As a result of the reduced total sheet resistance and the thin electrode layer, a film capacitor provided with the capacitor film according to the invention has good electrical properties in conjunction with an optimal self-healing behavior, which results in an increased life expectancy.